1. Field of the Invention
The present invention relates to an electron gun for a cathode ray tube used in a projector tube, a color picture tube, an index tube or the like, for example, and a manufacturing method thereof
2. Description of the Prior Art
FIG. 1A of the accompanying drawings shows an example of an electron gun for a cathode ray tube according to the prior art. FIG. 1B shows a potential diagram of the apparatus of FIG. 1A.
This conventional electron gun is of a unipotential type in which first to fifth grids G.sub.1 to G.sub.5 serving as acceleration and converging electrodes are disposed on the same axis (Z axis) in alignment with a cathode K from which electron beams are emitted, as shown in FIG. 1A. The electron beam emitted from the cathode K is converged on a phosphor screen (not shown) by action of a pre-focus lens formed of the second and third grids G.sub.2 and G.sub.3 and a main lens formed of the third to fifth grids G.sub.3 to G.sub.5. These cathode K and first through fifth grids G.sub.1 to G.sub.5 are fixed to a beading glass by melt bonding and assembled unitarily. The first to fifth grids G.sub.1 to G.sub.5 are made of a metal such as a stainless steel or the like.
An example of the above conventional electron gun, however, encountered with the problems which follow:
In the arrangement of the above electron gun, a displacement tends to occur in concentricity among the electrodes, in particular, the third to fifth grids G.sub.3 to G.sub.5 so that an electron beam is shifted from the axis to cause a so-called defocusing.
Further, a potential difference among the electrodes occurs in a step-wise fashion so that discharge tends to occur among the third to fifth grids G.sub.3 to G.sub.5. Furthermore, a spherical aberration of a lens system becomes large, which makes a spot diameter of electron beam large.